How to manipulate the value of y in the actuator motion via python?
import bge
cont = bge.logic.getCurrentController()
cont.actuators["Motion"].dLoc = (0, 0.1, 0)
cont.activate(cont.actuators["Motion"])


There are 3 important things you are missing.


A link to the python API which tells you what all the field names are https://docs.blender.org/api/2.79/bge.types.KX_ObjectActuator.html?highlight=actuator#bge.types.KX_ObjectActuator.dLoc
Setting the entire Vector instead of just 1 channel
Activate the actuator afterwards (you can also deactivate it by replacing "activate" with "deactivate")